1. Field of the Invention
The invention generally relates to wireless-access telecommunications systems such as mobile telephone, interactive pager, commercial-use radio communications systems, etc. and more particularly to adaptive array antenna systems used in such mobile radio communications systems.
2. Description of the Prior Art
An illustrative example of wireless-access telecommunications systems is cellular or mobile radio-telephone systems. In mobile telephone systems, frequency-division multiple access (FDMA) is used to maximize capacity by splitting available bandwidth into separate channels, which are assigned to respective cell zones. In each cell zone (or each radio channel), time-division multiple access (TDMA) is used to expand the channel capacity by permitting the base station (or radio cell) of the cell zone to serve a plurality of radio-telephones within the cell zone. An alternative capacity-expanding technique, known as code-division multiple access (CDMA) has been proposed to further expand capacity. CDMA allows reuse of the same radio-frequency spectrum in adjacent cells by the adjacent cells using codes noncorrelating with each other for spread spectrum modulation.
Since base stations typically uses adaptive array antenna, studies on directivity control techniques for the adaptive array antenna have been made as one of disturbance wave elimination techniques. In conventional antenna directivity control techniques for the adaptive antenna array, the directivity of an array antenna of a base station is controlled by directing the peak of the directivity to the direction of arrival or the location of a signal source (i.e., a mobile station) associated with the current time slot and by directing a null point of the directivity to the direction of an incoming disturbance wave. Some subspace-based algorithms, which involve covariance eigen-analysis, have been proposed as the direction-of-arrival estimation techniques used in directivity control. Two of such algorithms are well known as MUSIC (multiple signal classification) and ESPRIT (estimation of signal parameters via rotational invariance techniques).
The present invention relates to an array antenna directivity control method and system based on such a subspace-based algorithm and to a radio communications system incorporating the same.
However, in order to enable higher rate data transmissions such as image data transmission, the directivity of array antenna has to be controlled with a higher precision than conventional directivity control techniques can achieve. For this purpose, the sensitivity of reception of signals used in a direction-of-arrival estimator has to be raised.
It is therefore an object of the invention to provide a method of and a system for controlling directivity of an array antenna with a raised precision thereby to enable a higher rate data transmission.
The elements of array have to be increased in number with a growth in the number of mobile radio-telephones. This involves not only an increase in the area of the array but also an increase in processing time for the direction-of-arrival estimation, which makes real-time directivity control difficult.
It is another object of the invention to provide a method of and a system for controlling directivity of an array antenna having an increased number of elements or sensors in real time thereby to enable service to a larger number of mobile radio-telephones.
According to one aspect of the invention, the direction-of arrival estimation by means of a subspace-based algorithm such as MUSIC (multiple signal classification) or ESPRIT (estimation of signal parameters via rotational invariance techniques) is formed by using signals the bandwidths of which are narrower than those of communication signal waves (hereinafter, expressions like xe2x80x9can XX signal wavexe2x80x9d are used to mean xe2x80x9can RF carrier-modulated version of an XX signal). The narrower-bandwidth signals may be obtained by each mobile telephone transmitting a modulated version of a lower-transmission rate test (or estimation) signal as well as a communication signal wave or by band-pass filtering the received communication signal waves in a base station.
Using narrower-bandwidth signals causes the noise bandwidth to be the narrower, the correlation between the received signals to be reduced and the influence of changes in the characteristics of transmission path due to, e.g., fading to be reduced. This enables a higher-precision direction-of-arrival estimation, causing the array antenna to have a sharp directivity and resulting in raised sensitivities of both the base station and the mobile stations.
According to another aspect of the invention, the direction-of-arrival estimation by means of a subspace-based algorithm is formed by using a reduced number of received signals for the direction-of arrival estimation. The number of direction-of-arrival estimating signals is reduced by discarding a part of the received signals, by reducing the number of signals processed at a time (i.e., multiplexing the received signals by the sub-array and forming a direction-of arrival estimation for each sub-array, or by using an additional array antenna which has a reduced number of elements and is dedicated for the direction-of arrival estimation.
Using a reduced number of signals for the direction-of-arrival estimation enables a real-time direction-of-arrival estimation.
The first aspect and the second aspect of the invention are preferably used in combination. Doing this enables the directivity control of an array antenna to be achieved with a raised precision in real time. Accordingly, the sensitivities of both the base station and the mobile telephones are so enhanced that a large amount of data can be transmitted at a high speed as required in image data transmission.